In many hydrocarbon conversion processes, for example the conversion of methane to syngas, environments are encountered that have high carbon activities and relatively low oxygen activities. High temperature reactor materials and heat exchanger materials used in such processes can deteriorate in service by a very aggressive form of corrosion known as metal dusting. Metal Dusting is a deleterious form of high temperature corrosion experienced by Fe, Ni and Co-based alloys at temperatures in the range, 350-1050° C. in carbon-supersaturated (carbon activity >1) environments having relatively low (about 10−10 to about 10−20 atmospheres) oxygen partial pressures. This form of corrosion is characterized by the disintegration of bulk metal into powder or dust.
Although many high temperature alloys are designed to form an in-situ surface film of chromium oxide (Cr2O3) in low oxygen partial pressure environments, the nucleation and growth kinetics of this oxide are often not fast enough to prevent carbon intrusion in highly reducing carbon-rich environments with carbon activities in excess of unity. Furthermore, the formation of a Cr2O3 film provides initial protection against carbon ingress. The alloy is protected from carbon ingress in as much as the carbon does not migrate through the oxide film. However, the presence of defects and differential thermal contraction between the alloy and an oxide during oxide film growth could induce stresses that may result in rupture of the oxide film. Such local rupture of the oxide film would lead to carbon migration into the steel.
Methodologies disclosed in the literature for controlling metal dusting corrosion involve the use of surface coatings and gaseous inhibitors, for example H2S. Coatings can degrade by inter diffusion of the coating constituents into the alloy substrate. Thus, while coatings are a viable approach for short-term protection, they are generally not advisable for a long term service life of twenty years or more. Inhibition by H2S also has two disadvantages. One is that H2S tends to poison most catalysts used in hydrocarbon conversion processes. Secondly, H2S has to be removed from the exit stream which can substantially add to process costs.
U.S. Pat. No. 6,692,838 to Ramanarayanan et al. discloses compositions resistant to metal dusting and a method for preventing metal dusting on metal surfaces exposed to carbon supersaturated environments. The compositions comprise (a) an alloy, and (b) a protective oxide coating on the alloy. The alloy comprises alloying metals and base metals, wherein the alloying metals comprise a mixture of chromium and manganese, and the base metal comprises iron, nickel, and cobalt. U.S. Pat. No. 6,692,838 is incorporated herein by reference in its entirety.
A need exists for an advanced alloy composition that is resistant to metal dusting corrosion in low (about 10−10 to about 10−20 atmospheres) oxygen partial pressure and carbon-supersaturated (carbon activity >1) environments. Ideally, such an advanced alloy composition would be capable of rapidly forming an outer protective oxide film to block carbon transfer while growing an adherent inert oxide film slowly to act as a diffusion barrier to carbon ingress.